This invention in general relates to photographic projection printing and in particular to the use of an optical system employing an aperture mask in combination with a flying spot type enlarger for controlling tones and reducing certain visual artifacts caused by fringing.
Photographic masking is a technique well-known from early on in the art of projection printing or enlarging. Its essential form involves placing a mask, something which will reduce the quantity of light, in the path of the light forming an image to modify the photographic result otherwise obtainable. When the mask is used to selectively shade different portions of a projected image of a negative to reduce exposure, the masking technique is referred to as dodging, and when used to selectively increase the density of different portions by giving extra exposure, the technique is referred to as burning-in. Thus dodging and burning-in are techniques which can be used to adjust printing exposure from area to area over a negative to compensate for an imbalance of tones which may be present in the negative and thus provides a means for increasing the detail registered in highlights and shadows.
A more elaborate use of masks, also well-known, is for tone control. Here, a mask is employed where it is desired to print a negative with a wide range of tones, i.e., high-contrast, onto a print material having a dynamic range insufficient to capture the range of tones present in the negative. The mask is made as a full-toned, but weak and soft, positive contact print from the hard negative on another film. This is then accurately registered and bound into contact with the negative. When this sandwich is projected, the mask tends to cancel out the negative and has the effect of considerably reducing the contrast of the negative thereby matching its tonal range to that of the print material.
Tone control masks can be either sharp or unsharp. Both sharp and unsharp masks are made by contact printing the negative on print material, but, in the case of the unsharp mask, the negative and print material are typically separated by a spacer so that the mask comes out defocused or with blurred outlines. A negative printed with an unsharp mask appears more sharp than one printed with a sharp mask because there is a tendency for the blurred outlines of the unsharp mask to cancel the slight blurring of outlines present in the negative. Thus, with the unsharp mask, both tone control and higher apparent sharpness are achieved whereas, with the sharp masks, only tone control occurs.
Both sharp and unsharp masks can be mimicked if use is made of a flying spot scanner as a source for illuminating the negative instead of a continuous source like those used in conventional enlargers. With a flying spot scanner, an electron beam moves over the faceplate of a cathode ray tube in a regular pattern to produce a light spot which exposes the negative in point by point fashion. The light emerging from the negative can be continuously monitored by a phototube which in turn can influence the brightness of the light spot. In this manner, an automatic and simultaneous shading and overprinting of various parts of the negative is done according to the local density of the negative. This reduces contrast of the negative in the same way as an unsharp mask because the negative is printed with a light source that is in effect a luminous positive image of the negative. Thus the flying spot scanner performs all the functions of an unsharp mask except that the "light" mask is formed automatically during the printing process and therefore requires no preparatory work or processing.
However, the finite size of the scanning light spot, which determines the unsharpness and hence detail control of the "light" mask, leads to a fringing effect which is characteristic of electronic printing with a moving spot. As the spot scans the boundary of the image from light to dark, the light reaching the monitoring photocell does not change suddenly but gradually. With the spot halfway across the boundary, the light falling on the phototube is halfway between that for the bright and for the dark areas. Because the feedback circuit, which regulates spot intensity, reacts instantly, the boundary of the shadow area begins to receive more exposure than the shadow region just traversed by the spot. Similarly, the edge of the highlight area gets less exposure than the region covered by the spot immediately afterwards. This leads to a dark fringe at the border of the shadow area in the print and a light fringe at the border of the highlight area where it is particularly noticeable.
Thus, while the electronic printing is advantageous for a variety of reasons, it characteristically produces fringing effects or visual artifacts which are highly undesirable in the final image because they are visibly noticeable. Consequently, it is a primary purpose of the present invention to provide projection printing apparatus for reducing fringing artifacts in transparency prints.
It is another purpose of the present invention to provide an optical system employing an aperture mask for use in conjunction with a flying spot illumination type enlarger by which fringing effects or artifacts are made less noticeable in transparency prints.
Other objects of the invention will in part be obvious and will in part appear hereinafter. The invention accordingly comprises the apparatus possessing the construction, combination of elements, and arrangement of parts which are exemplified in the following detailed discription.